For example, a secondary battery that is used by repeatedly charging has become essential in recent social life due to the popularization of hybrid vehicles, electric vehicles, mobile phones, and the like.
Although a secondary battery has evolved to have a large battery capacity, charging/discharging in a repeating manner gradually increases an internal change and inhibits sufficient charging/discharging, thereby reducing the life of some secondary batteries.
Therefore, especially when the secondary battery is used in a vehicle, there is a risk that the vehicle suddenly becomes undrivable due to degradation of the secondary battery. It is thus desired to use the secondary battery after determining whether the secondary battery is degraded.
As such, methods of determining degradation of the secondary battery have conventionally been developed.
For example, NPL 1 set forth the below discloses degradation determination by measuring internal resistance corresponding to degradation of a lithium-ion battery.
Also, PLT 1 set forth below discloses detection of a state of a lithium-ion secondary battery by applying an AC voltage and/or an alternating current at a particular frequency to the lithium-ion secondary battery. PLT 2 set forth below discloses a method of measuring an AC impedance of a non-aqueous electrolyte secondary battery at a predetermined frequency and estimating reversible capacity of the battery from a relational expression of the AC impedance and the reversible capacity allowing charging/discharging (a battery capacity that allows charging/discharging).
Also, PLT 3 set forth below discloses: deriving voltage-current characteristics of the lithium-ion battery; deriving an open-circuit voltage (Open-Circuit Voltage: OCV) of the lithium-ion battery based on the voltage-current characteristics thus obtained; estimating a charging capacity (State Of Charge: SOC) of the lithium-ion battery by employing current integration or the like; and determining deposition degradation based on changes in the OCV and the SOC.
PLT 4 set forth below discloses determination on degradation based on information about a change in a voltage obtained in a diagnostic mode for continuously discharging and charging the lithium-ion battery at a constant electric power.
Further, PLT 5 set forth below discloses: in charging the lithium-ion battery by employing a constant current and constant voltage scheme, setting a charging current at C0/(20 hours) or less, provided that the C0 represents a nominal capacity of the battery; obtaining time t from when a charging voltage during charging with the constant current reaches a predetermined voltage Vs to when the charging voltage reaches an upper limit Vc of the charging voltage; and estimating the capacity (Ce) of the lithium-ion battery from the estimated capacity ratio Ce/Co by using a relational expression Ce/C0=At+B (A, B=const); and determining degradation.
A degradation determination method described in PLT 6 set forth below is as follows. That is, when the secondary battery such as the lithium-ion battery, a nickel-cadmium battery, or a nickel-metal hydride battery is connected to a charging device, a type of the secondary battery is detected and, based on a voltage of the battery, constant-current charging processing commences. During the constant-current charging processing, when the voltage of the battery reaches a reference voltage corresponding to the type of the battery, a controller starts measuring constant-current charging time. Depending on a charging control method corresponding to the type of the secondary battery being used, when the constant-current charging is switched over to constant-voltage charging, or when −ΔV is detected, the measurement of the time ends. The control unit compares the constant-current charging time obtained by the measurement and a constant-current charging time of a battery having a state of charging capacity of a brand new battery.